1. Field of the Invention
This invention relates generally to apparatus for practicing the printing process commonly known as silk screen printing and more particularly to an improved screen frame assembly for mounting the printing screen used in this printing process.
2. Prior Art
The printing technique commonly called silk screen printing and referred to herein simply as screen printing, is well understood and widely used for a great variety of printing applications. Screen printing involves the use of a fine mesh printing screen which has been processed to block selected holes in the screen in such a way that the blocked and open screen holes define the image to be printed. The printing screen is stretched taut across a screen frame which is then placed over the work surface to be printed with the underside of the screen in contact with the surface. A screen printing ink is wiped across the upper side of the screen to force the ink through the open screen holes onto the work surface and thereby reproduce the screen image on the surface.
A printing screen is mounted on a screen frame in either of two basic ways. According to one way, the screen is stretched taut across and secured along its edges to a separate stretch frame. The stretch frame and its taut screen are placed over a screen frame, and the screen edges are secured to the side members of the screen frame. The stretch frame is then removed from the screen, leaving the tautly stretched screen on the screen frame. Another way of mounting a printing screen involves the use of a screen frame having screen anchor members, such as rollers, draw bars, or the like. The screen edges are secured to these anchors which are then rotated, in the case of screen anchor rollers, or moved laterally outward, in the case of draw bars, to stretch the screen edgewise across the screen frame.
The prior art is replete with a vast assortment of printing screen frames. By way of example, U.S. Pat. No. 4,409,749 discloses a screen frame having rollers to which the screen edges are secured and which are rotatable to stretch the screen across the screen frame. U.S. Pat. No. 3,553,862 discloses a screen frame having draw bars to which the screen edges are secured and which are movable laterally to tension the screen on the screen frame. In U.S. Pat. No. 3,167,004, a stencil screen is initially stretched taut and then secured to a screen frame.
A printing screen is fixed to a screen frame in various ways. In U.S. Pat. Nos. 3,553,862 and 4,409,749, for example, the screen is releasibly gripped by screen anchors on the frame. In U.S. Pat. No. 3,167,004, the screen is attached to the frame by a fushion bond. U.S. Pat. No. 4,702,783 discloses a process for adhesively bonding a pre-stretched printing screen to a screen frame using a cyanoacrylate based adhesive and catalyst.